1. Field of the Invention
The present invention relates to a toner for electrostatic latent image developing and manufacturing method of same for developing electrostatic latent images in electrophotography, electrostatic recording, and electrostatic printing.
2. Description of the Related Art
Conventional toners for electrostatic latent image developing for use in developing electrostatic latent images in electrophotography, electrostatic recording, and electrostatic printing are manufactured by a so-called dry pulverization method wherein a pigment such as carbon black and the like is thoroughly mixed by kneading in a thermoplastic resin so as achieve a uniform dispersion, which is subsequently pulverized to a powder of required particle size, i.e., toner, by a suitable fine pulverization device.
In recent years, granulation by wet process as represented by dispersion polymerization, emulsion dispersion and the like which allow production of fine resin particles of smaller and relatively even size have been of interest in place of pulverization methods in view of manufacturing cost reduction and improved image quality. Hereinafter, the method for producing particles by wet process, namely, the method for producing particles in solution as described above shall be referred to as "wet granulation method."
The dispersion polymerization method produces particles by dispersing and polymerizing polymer constituents such as polymeric monomers, polymerization initiators, coloring agents and the like in a dispersion fluid.
The emulsion dispersion method forms droplets of resin solution by dissolving or dispersing binder resin and coloring agent in a suitable organic solvent to produce a colored resin solution which is added to an aqueous dispersion fluid and aggressively mixed. The droplets are heated to remove the organic solvent.
The wet granulation method can correspond well to higher quality images because toner particles of small size can be readily formed. Furthermore, yield is excellent by this method.
Toner produced by the wet granulation method exhibits excellent charging characteristics under normal temperature and humidity, but was found to have inadequate characteristics when charging characteristics were measured after long-term storage under high temperature and high humidity conditions. Therefore, when the toner is subjected to conditions of high temperature and high humidity during shipment and transport, adequate charging characteristics are not exhibited and image quality deteriorates.
Normally, in the aforesaid wet granulation method, a dispersion stabilizer is added to the dispersion fluid to stabilize the dispersion state of the droplets by preventing flocculation of said droplets dispersed in the dispersion fluid.
Although various dispersion stabilizing agents are known, it is particularly desirable to use a calcium-containing inorganic salt with lower water-solubility such as calcium phosphate and the like because of its excellent dispersion stabilization characteristics and the ease with which the dispersion stabilizer can be removed.
Toners produced by the wet granulation method using the aforesaid inorganic salt comprising calcium as a dispersion stabilizer are disadvantageous in that they do not maintain adequate charge amount, such that a large amount of the toner is inadequately charged, charging characteristics deteriorate after storage at high temperature and high humidity, and the charge is reduced during printing.